From time to time our client does not receive email from us.
We got an NDR with the following:
Your message did not reach some or all of the intended recipients.
The following recipient(s) cannot be reached:
The e-mail system was unable to deliver the message, but did not report a specific reason. Check the address and try again. If it still fails, contact your system administrator.
#2.0.0 smtp;221 Service closing transmission channel closing connection
I've checked http://mxtoolbox.com/blacklists.aspx
it shows our SMTP's IP is not black listed.
We have Exchange 2003, the client's mx's port 25 respond with "220 SMTP Proxy Server Ready"
What is the most probable cause of the problem?